1. Field of the Invention
The present invention relates to a document feeder, and a method of document reading and more particularly to a document feeder which picks up stacked documents one by one and feeds each document on an image reading position over a stationary image reading device at a constant speed.
2. Description of Related Art
Recently, various types of document feeders adopting a method in which the image of a document separated from a stack and fed to a reading position is read by a scanner, namely, a document-feeding image reading method have been provided. In such a document feeder, generally, separating/feeding means (rollers) is so structured to rotate following a document which is transported by transport means which is located downstream. Accordingly, when the trailing edge of the document passes the separating/feeding means, the load on the transport means changes, which changes the transport speed, thereby resulting in noise in the read image.
In order to solve this problem, conventionally, either of the following measures have been taken: a document is fed over the reading position once so that the trailing edge of the document certainly passes the separating/feeding means, and thereafter, the document makes a switchback and while being fed on the reading position again, the image is read; and the pressure for separating documents in the separating/feeding means is set low so that a change in load which occurs when the trailing edge of a document passes the separating/feeding means will be small.
However, in the former, because image reading is carried out after a switchback of a document, the copying productivity is low. In the latter, because the separating pressure is low, it is likely that a plurality of documents are fed at a time.